A motivation for removing the airwave in marine CSEM data is as follows: In marine CSEM or SBL one looks for thin resistive layers in conductive sediments by using a horizontal electric dipole (HED) source that emits strong electromagnetic fields into the surroundings. The source is towed close to the seabed and the low-frequency signals are recorded by receivers that are situated on the seabed (Eidesmo et al., 2002). The principle behind marine CSEM/SBL is simple; hydrocarbons in the subsurface appear to low-frequency electromagnetic fields as thin resistive layers within more conductive sediments. Hydrocarbons can thus in principle be directly detected by transmitting electromagnetic fields into the subsurface and recording the returning signal that has been guided in the thin resistive layer. However, in shallow water, the lateral field propagation in the air halfspace becomes significant, and may dominate the recorded signals at the receivers.
The signal that travels through air is often referred to as the source-induced “airwave” or “air-response.” Since the subsurface responses are present in the data even if they are dominated by the air-response, the air-responses can be included in inversion schemes. However, it can often be advantageous to separate the air-response from the subsurface data in order to simplify the interpretation and to make the migration and inversion schemes more efficient.
Several methods have been proposed to separate the air-response from the subsurface signal. One proposed solution is to generate synthetic data in a two-halfspace model with seawater and air in order to subtract the synthetic data from the real data for the relevant source-receiver geometries (Lu et al., 2005). Another method is to apply electromagnetic field decomposition into upgoing and downgoing components just below the seabed as suggested by Amundsen et al., (2006). Instead of, or in addition to, decomposing the electromagnetic field into upgoing and downgoing field components, the field can be split into a transverse electric (TE) part and a transverse magnetic (TM) part.
It is known that the subsurface response from a thin resistive layer is due to a TM-polarized refracted response whereas the air-response is dominated by a TE-mode lateral wave (cf. Løseth, 2007). That the TM mode is sensitive to a thin resistive layer is one of the main points in Eidesmo et al. (2002). Since the vertical electric field component from a HED is a pure TM mode, one could measure this component in order to avoid the TE-mode air-response. The air-response could likewise be avoided by using a vertical electric dipole (VED) since this source configuration generates TM modes only. However, since the HED is able to induce larger subsurface responses than the VED, and since horizontal field components are more easily measured than vertical field components with present surveying techniques, the air-response removal from horizontal field components due to an HED source is an issue of great interest in hydrocarbon exploration with the marine CSEM/SBL method. Although it is common in conventional surveying to collect inline data only, it is known that access to both inline and broadside data can provide additional information (e.g. Eidesmo et al., (2002).